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Home , Austrocedrus, Vegetation

Temperate Coniferous

Richard H Waring Volume 2, The Earth system: biological and ecological dimensions of global environmental change, pp 560–565

Edited by

Professor Harold A Mooney and Dr Josep G Canadell

in Encyclopedia of Global Environmental Change (ISBN 0-471-97796-9)

Editor-in-Chief

Ted Munn

 John Wiley & Sons, Ltd, Chichester, 2002 and sunken stomata and a large proportion of their stems Temperate Coniferous consists of sapwood that serves as a storage reservoir, as Forests well as a conduit for transport of water from roots to .

Richard H Waring GEOGRAPHIC DISTRIBUTION Oregon State University, Eugene, OR, USA Today, temperate coniferous forests cover app- roximately 2.4 ð 106 km2 (Melillo et al., 1993; Landsberg Temperate coniferous forests include representatives of the and Gower, 1997). dominate the montane forests most massive forms of terrestrial life on Earth. In the in , , and China. Smaller areas of tem- coastal redwood ( sempervirens) forests of northern perate conifers are located in montane regions of Korea, California, may reach heights >100 m and accumu- Japan, Mexico, Nicaragua, and Guatemala. In Europe, the late biomass in excess of 3000 t ha1 . In the more extensive distribution of native and introduced conifers has been Douglas fir (Pseudotsuga menziesii) forests of the Pacific expanded into areas more climatically favorable to hard- Northwest, USA and Canada, biomass in older stands fre- woods. Pines (Pinus), represented by more than 90 , quently reaches 1000 t ha1 , with an additional 500 t ha1 are the most widely distributed and their range has of standing dead and fallen trees. Above-ground biomass increased by widespread planting in the Southern Hemi- ranges widely, however, as does above-ground growth. sphere. Other important genera include the firs (Abies), Temperate coniferous forests produce an average of about spruces (Picea), hemlocks (Tsuga), false-hemlocks (Pseu- 5tha1 1 of above-ground biomass but in climat- dotsuga), larches (Larix), (), cedars ically diverse areas production may range from <1to (Chamaecyparis, Thuja, Libocedrus) and (Junipe- >20 t ha1 year1 (Runyon et al., 1994). rus). In the yews (Taxus), the are not borne in Because of their high growth capabilities, favorable wood cones but are enclosed within a fleshy structure. Addi- structural properties and columnar form, temperate conif- tional conifer genera (Araucaria, , Austrocedrus) erous forests play a disproportionate role in meeting global are present in the Southern Hemisphere, where they were demands for lumber and fiber. As a consequence, the geo- once more widely distributed (Axelrod et al., 1991). graphic range of temperate conifers has been extended beyond the native ranges of many species. In this contribution we focus on the attributes of tem- ECOLOGICAL DISTRIBUTION perate evergreen coniferous forests that distinguish them from their major competitors, hardwoods. This The potential ecological niche of temperate conifers falls differentiation should help us anticipate the future role and within regions of >250 mm in annual precipitation that experience subfreezing conditions, down to, but not below importance of temperate coniferous evergreen in relation to ° rising levels of air pollution, atmospheric carbon dioxide 45 C, which is the limit for supercooling of water. Only boreal species, which include some pines and spruces, (CO2 ), and changing climate. are adapted to temperatures below this limit (Waring and Running, 1998). In regions where precipitation is well EVOLUTION distributed throughout the growing , deciduous hard- woods usually dominate over temperate conifers. Following Conifers bear naked seeds, unlike angiosperms, and dif- major disturbances, however, conifers can become estab- fer from other gymnosperms by having the seeds arranged lished and achieve temporary dominance (Landsberg and inside cones. During the Carboniferous period (300 million Gower, 1997). ago), conifers represented a minor group of gym- In the Pacific Northwest region, where summer nosperms, but they evolved rapidly as conditions became is common, the situation is reversed and many long-lived drier and colder during the Permian (286–240 million years conifers replace earlier established hardwoods (Waring and ago). Temperate coniferous forests were most widely dis- Franklin, 1979). To explain these shifts in dominance tributed during the Tertiary period (90–15 million years requires an appreciation of how resources are captured by ago), with coastal redwood ()and temperate evergreen conifers throughout the year. dawn redwood () distributed throughout both Eurasia and North America. Today, conifers are the domi- nant representatives of gymnosperms with about 50 genera PHYSIOLOGICAL AND MORPHOLOGICAL and 550 species, predominantly in the Northern Hemisphere ADAPTATIONS OF CONIFERS (Raven et al., 1986). Conifers are well adapted to arid and cold conditions. The Conifer leaves are more clumped and narrower in cross- surfaces of needle-shaped leaves are protected by waxes section than the foliage of most angiosperms, and the 2 THE EARTH SYSTEM: BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE branch structure is much less extensive. These proper- The evolutionary advantages that conifers have on infer- ties allow light to penetrate more deeply through conifer tile can place them at a disadvantage on more fertile canopies than through hardwood forests with comparable sites. On infertile soils, most available nitrogen (N) is in C1 area. As a result, the maximum surface area of conifer the form of ammonium (NH4 ), which conifers take up 2 1 foliage, often expressed as m of (projected) leaf area per selectively in preference to nitrate–nitrogen (NO3 ). As m2 of ground surface, may reach 10–12, while hardwood nitrate–nitrogen becomes more available, conifers are dis- forests rarely exceed half these values. In addition, conifer advantaged in competition with hardwoods (Smirnoff and canopies reflect only about 5–10% of intercepted solar radi- Stewart, 1985). ation, whereas hardwoods reflect 15–25%. In combination, these differences in canopy properties permit coniferous forests to absorb a greater proportion of incoming solar RESPONSES OF TEMPERATE radiation than forests composed of hardwoods (Jones 1992). CONIFEROUS FORESTS Although conifers absorb a larger fraction of intercepted radiation than hardwoods, they are less prone to suffer damage from elevated temperatures because the needle- Leaf litter, wood, and root materials produced by ever- shaped foliage dissipates heat efficiently, even under unven- green conifers usually contain twice the amount of carbon tilated conditions. Consequently, conifers conserve water (C) in relation to nitrogen found in corresponding materi- under conditions where broad-leaf species must transpire at als produced by deciduous angiosperms. As a result, the higher rates, wilt, or shed foliage to prevent temperatures decomposition of coniferous litter is usually 3–4 times from rising above 45 °C, when enzymes begin to become slower than hardwood litter, leading to a greater accumu- unstable. lation of floor litter under conifers (Figure 1). Another foliage characteristic that distinguishes most With time, as litter decays, soils under coniferous forests conifers from other evergreen vegetation is the duration maintain high C–N ratios and serve as storage sites for that leaves remain functional. Conifers generally hold some amounts of carbon that far exceed above-ground biomass, foliage for 3–5 years, although a few species maintain and have turnover times of centuries and millennia. This functional leaves for 30–40 years (Landsberg and Gower, 1997). The fact that temperate evergreen conifers can main- 100 tain dense, needle-leaf canopies, composed of multiple-age MRT = 20 10 classes of foliage, affects many ecosystem processes includ-

) 80 ing the capture of water, nutrients, and pollutants from the 1 − air, and the rates of decomposition, nutrient release, and 1 organic matter accumulation. 60 Anatomically, long-lived foliage is characterized by an 4 epidermis covered with a thick waxy cuticle, above one 2 3 or more layers of compactly arranged, thick-walled cells, 40 which in turn surround a structurally reinforced vascular system. The result is that nutrient concentrations in conifer

Forest floor mass (t ha Forest 4 2 needles may be only half those in deciduous hardwood 20 5 leaves of equivalent mass. Lower nutrient concentrations 6 in individual leaves translate into lower demand, yet long- 7 lived foliage provides a greater store of nutrients available 0 24681012 for transfer during peak growth periods than is available in Litterfall (t ha−1 year −1) deciduous hardwoods. In addition, in environments where mineral weather- Figure 1 Average forest floor mass plotted against lit- ing rates are slow and nutrient availability limited, ever- ter-fall for (1) boreal needle-leaved , (2) boreal green conifers can take up nutrients from the soil as long broad-leaved deciduous, (3) temperate needle-leaved ever- green, (4) temperate broad-leaved evergreen, (5) tem- as conditions remain above freezing. can perate broad-leaved deciduous, (6) tropical broad-leaved also continue under these conditions. As a result of nutri- evergreen, and (7) tropical broad-leaved deciduous trees. ent uptake and photosynthesis during the dormant season, Assuming the forest floor is at steady state, the average stored reserves (starch, sugars, amino acids) are available to mean residence time (MRT, years) can be calculated as activate symbiotic fungi on tree roots that efficiently scav- forest floor mass/litterfall mass. The lines indicate that for- est floor litter of temperate needle-leaved evergreens take enge nutrients. In contrast, deciduous trees, which also have about 15 years to turnover whereas other temperate hard- symbiotic root fungi, are restricted to taking up nutrients woods turnover in <4 years. (Reproduced from Landsberg when foliage is present. and Gower, 1997) TEMPERATE CONIFEROUS FORESTS 3 property of coniferous forest , combined with formed. Ozone is a highly reactive oxidizing agent that their potential rapid growth, makes them an attractive diffuses through stomatal pores on leaf surfaces when they vegetation type to manage for the sequestration of large are open and injures the photosynthetic machinery inside. amounts of atmospheric carbon dioxide (CO2). Over a single growing season, conifer foliage suffers less damage than deciduous hardwoods because gas exchange Hydrology rates through stomata are only about half those of decid- uous hardwoods for an equivalent surface area (Reich The potential of evergreen coniferous forests to maintain and Amundson, 1985). With continued exposure, how- dense, well ventilated canopies throughout the year sig- ever, older foliage on conifers sustains accumulated damage nificantly increases the annual water vapor transfer from and is shed prematurely (McLaughlin, 1985), leading to their canopies, relative to deciduous forests. As a result, reduction in tree vigor and a greater likelihood of disease evergreen coniferous forests reduce stream flow by an aver- and insect outbreaks (Waring and Running, 1998). age of 15–25% in comparison to deciduous hardwoods in similar environments (Waring and Schlesinger, 1985). The dense, finely dissected canopies of evergreen conifers also RESPONSE OF TEMPERATE CONIFEROUS have the propensity to condense water (and pollutants) from FORESTS TO CLIMATIC CHANGE AND clouds, leading to fog drip, which can increase the effec- INCREASING ATMOSPHERIC CO2 tive precipitation at high elevations substantially in windy environments (Harr, 1982). We might expect temperate coniferous forests to be favored Water stored in the sapwood of conifer trees can serve over deciduous hardwoods if climatic conditions during the as a temporary buffer against drought. During the day, dormant season become more favorable and those during this reservoir may contribute up to a third of the water the growing season less so. Thus, if minimum temper- transpired, with refilling taking place during the night. In atures rise during the winter, the constraints exerted by general, the ratio of sapwood mass to leaf area increases subfreezing and suboptimal temperatures on photosynthe- in conifers with harshness of the environment (Waring and sis will be reduced (Runyon et al., 1994; Landsberg and Running, 1998); the total volume of sapwood determines Waring, 1997). The relative importance of warmer condi- the reservoir of water. Thus, a 50 m tall Douglas fir forest tions during the winter can be assessed by considering the with about a quarter of the stem composed of sapwood seasonal shifts in potential radiation available at different may hold 30 mm of stored water whereas a 15 m tall latitudes, aspects, and slopes to evergreen conifers (Run- pine forest, with stems composed largely of sapwood, can ning et al., 1987; Waring and Running, 1998). If warming hold 20 mm of stored water (Waring and Running, 1978; is combined with more frequent storms, without necessarily Waring et al., 1979). In a few species, such as redwood, any increase in total precipitation, the associated increase in the voluminous heartwood also plays an important role as cloud cover may substantially reduce solar energy available a reservoir. to melt snow or drive photosynthesis. On the other hand, if climatic warming extends through- out the year, the growing season will be lengthened. A RESPONSE OF TEMPERATE CONIFEROUS FORESTS TO AIR POLLUTION higher frequency of storms in this case could moderate summer temperatures by allowing vegetation to maintain In clean environments, nitrogen deposition is usually transpiration at near maximum rates and reduce the frac- <5kgha1 year1 whereas in heavily polluted areas, nitro- tion of energy going into heating the air (Figure 2). More gen additions exceed 50–70 kg ha1 year1. Much of this frequent wetting of surface litter and soil would also fos- excess nitrogen ends up as nitrate, which is easily leached ter more rapid decomposition and release of nutrients, while in soil solution. The negatively charged nitrate ions extract reducing the likelihood of forest fires. This climatic scenario calcium, magnesium, and other positively charged nutrient would favor replacement of conifers by hardwoods. from soil cation (C) exchange sites. Because weathering To factor in the response of vegetation to rising atmo- rates are slow, the lost nutrients are often replaced with spheric CO2 concentrations is more of a challenge. In C1 positively charged acid radicals (H ) and toxic forms of general, increases in CO2 should result in elevated rates aluminum (AlC3). As a result, a nutrient imbalances may of photosynthesis because carbon dioxide is a limiting arise where the availability of nitrogen is in excess to that resource for all tree species. Moreover, a rise in CO2 should of other elements, creating conditions where become reduce photorespiration and increase the efficiency of more nutritious, and less resistant to attack from a variety photosynthesis, particularly if conditions become warmer. of organisms (Waring and Running, 1998). Because stomatal adjustments in most tree species main- In areas where hydrocarbons released by the combus- tain CO2 concentrations within their leaves at about 75% 3 tion of fossil fuels interact with sunlight, ozone (O3 )is of the value in ambient air, we might conclude that rising 4 THE EARTH SYSTEM: BIOLOGICAL AND ECOLOGICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE

FOREST MANAGEMENT OPTIONS 15.5 With increasing demand for forest products, and increasing 14.5 Leaf emergence appreciation of other values derived from forests, there is considerable controversy and concern over how forests, 13.5 on both public and private , are to be managed. C) ° 12.5 Temperate coniferous forests have tremendous value to human societies, so there is justified concern about their 11.5 management. Ancient coniferous forests have long been appreciated by many cultures as places of beauty, often with 10.5 religious significance. Many groves have been designated Diurnal ( range as sacred and others set aside as natural wonders to be 9.5 Last freeze preserved in parks. The value of forests for maintaining clear running streams and high quality drinking water 8.5 has also been long appreciated. Recently, modifying the Snowmelt complete structural characteristics of young coniferous forests to 7.5 create properties more typical of mature stands has gained −56.0 −42.0 −28.0 −14.0 0.0 14.0 28.0 42.0 56.0 acceptance as a means of creating more favorable Days after first leaf date for rare and endangered species commonly associated with extremely old forests. At the same time, ways of increasing Figure 2 Average daily surface temperature amplitude (maximum–minimum), measured before and after spring the amount of carbon stored in biomass and soils are being leaf emergence of local vegetation for 12 sites across the developed. These carbon-sequestering techniques affect site north-central United States. The rapidly warming spring preparation and harvests as well as the design, use, and temperatures generate progressively larger temperature recycling of forest products. amplitudes during the 2–3 week period prior to leafing out; It is clear that young plantations will play an increasing the amplitude is dampened, however, by leaf emergence as partitioning of incoming solar radiation is shifted as role in providing lumber and fiber: whether these planta- a result of cooling by rapidly transpiring vegetation. tions replace native species or reclaim marginal farmland, (Reproduced from Schwartz and Karl, 1990; Waring and however, will make a difference in the extent that and Running, 1998) animal biodiversities are affected. Similarly, the fraction of the occupied by plantations will alter the likeli- hood of fire, water runoff, and the potential for outbreaks atmospheric CO2 will result in a net reduction in transpi- of insects and diseases. ration per unit leaf area, and thus increased carbon gain The challenge associated with more active management per unit of water lost. This may be generally true for many for wood, water, biodiversity and carbon storage, is the hardwood species, but there is less evidence that conifer need to anticipate the extent to which disturbances may stomata respond similarly (Norby et al., 1999). In fact, in occur across broad landscape and ownership patterns. The one field experiment where ambient CO2 levels were arti- introductions of exotic organisms into native forests is a ficially elevated for more than two years, no shift in pine particular concern because exotic insects and pathogens can stomata behavior was recorded (Ellsworth, 1999). result in replacement of native trees and open the environ- If net photosynthesis were to increase significantly, ment to invasion by noxious weeds and other organisms. canopy leaf area index would reach a higher value, assum- General models that employ computer and satellite tech- ing sufficient nutrients were available. An increase in leaf nology to compare projected changes with those that can area would affect the water balance by increasing canopy be observed across broad and regions can pro- interception, evaporation, and transpiration under a stable vide a valuable aid to decision making. Remote sensing climate. On the other hand, if climatic conditions were technology by itself permits us to monitor the state of the to warm without a corresponding increase in precipitation, Earth’s vegetation and the extent of change in vegetative drought could reduce photosynthesis, growth, and canopy cover over the last three to four decades. leaf area. In summary, although changing patterns of climate and atmospheric CO2 concentration together are likely to have REFERENCES subtle effects on tissue biochemistry and the allocation of resources, forest growth and composition could be dra- Axelrod, D I, Kalin-Arroyo, M T, and Raven, P H (1991) History matically altered as a consequence of from of Temperate Vegetation in the Americas, Rev. Chil. Hist. Nat., outbreaks of fire, diseases, and insects. 64, 413–446. TEMPERATE CONIFEROUS FORESTS 5

Ellsworth, D S (1999) CO2 Enrichment in a Maturing Pine Forest: Runyon, J, Waring, R H, Goward, S N, and Welles, J M (1994) are CO2 Exchange and Water Status in the Canopy Affected? Environmental Limits on Net Primary Production and Light- Plant, Cell Environ., 22, 461–472. use Efficiency across the Oregon Transect, Ecol. Appl., 4, Harr, R D (1982) Fog Drip in the Bull Run Municipal Watershed, 226–237. Oregon, Water Resour. Bull., 18, 785–789. Smirnoff, N and Stewart, G R (1985) Nitrate Assimilation and Jones, H G (1992) Plants and Microclimate, 2nd edition, Cam- Translocation by Higher Plants: Comparative Physiology and bridge University Press, Cambridge. Ecological Consequences, Physiol. Plant, 64, 133–140. Landsberg, J J and Gower, S T (1997) Applications of Physio- Schwartz, M D and Karl, T R (1990) Spring Phenology: Nature’s logical Ecology to Forest Management, Academic Press, San Experiment to Detect the Effect on “green-up” on Surface Diego, CA. Maximum Temperatures, Mon. Weather Rev., 118, 883–890. Landsberg, J J and Waring, R H (1997) A Generalized Model of Waring, R H and Franklin, J F (1979) Evergreen Coniferous Forest Productivity Using Simplified Concepts of Radiation- Forests of the Pacific Northwest, Science, 204, 1380–1386. use Efficiency, Carbon Balance and Partitioning, For. Ecol. Waring, R H and Running, S W (1978) Sapwood Water Storage: Manage., 95, 209–228. its Contribution to Transpiration and Effect Upon Water Con- McLaughlin, S B (1985) Effects of Air Pollution on Forests, a ductance Through the Stems of Old-growth Douglas-fir, Plant, Critical Review, J. Air Pollut. Ctrl. Assoc., 35, 513–534. Cell Environ., 1, 131–140. Melillo, J M, McGuire, A D, Kicklighter, D W, Moore, III, B, Waring, R H, Whitehead, D, and Jarvis, P G (1979) The Contri- Vorsmarty, C J, and Schloss, A L (1993) Global Climate bution of Stored Water to Transpiration in Scots Pine, Plant, Change and Terrestrial Net Primary Production, Nature, 363, Cell Environ., 2, 309–317. 234–240. Waring, R H and Schlesinger, W H (1985) Forest Ecosystems: Norby, R J, Wullschleger, S D, Gunderson, C A, Johnson, D W, Concepts and Management, Academic Press, Orlando, FL. Waring, R H and Running, S W (1998) Forest Ecosystems: Anal- and Ceulemans, R (1999) Tree Responses to Rising CO in 2 ysis at Multiple Scales, 2nd edition, Academic Press, San Field Experiments: Implications for Future Forests, Plant, Cell Diego, CA. Environ., 22, 683–714. Raven, P M, Evert, R F, and Eichhorn, S E (1986) Biology of Plants, 4th edition, Worth, New York. Reich, P B and Amundson, R G (1985) Ambient Levels of Ozone FURTHER READING Reduce Net Photosynthesis in Tree and Crop Species, Science, 230, 566–570. Townshend, J, Justice, C, Li, W, Gurney, C, and McManus, J Running, S W, Nemani, R R, and Hungerford, R D (1987) Ex- (1991) Global Cover Classification by Remote Sensing: trapolation of Synoptic Meteorological Data in Mountainous Present Capabilities and Future Possibilities, Remote Sens. Terrain and its Use for Simulating Forest Evapotranspiration Environ., 35, 243–255. and Photosynthesis, Can. J. For. Res., 17, 472–483. VEMAP members (1995) Vegetation/ecosystem Modeling and Running, S W, Loveland, T R, Pierce, L L, and Hunt, E R, Jr Analysis Project (VEMAP): Comparing and (1995) A Remote Sensing Based Vegetation Classification Biogeochemistry Models in a Continental-scale Study of Ter- Logic for Analysis, Remote Sens. Environ., 51, restrial Ecosystem Responses to Climate Change and CO2 39–48. Doubling, Global Biogeochem. Cycles, 9, 407–437.